The present invention relates to a sensor element, especially a temperature sensor, according to the species defined in the main claim.
From German Application DE 196 51 454 A1, an electrical resistor is known, which is made by melting down of metal-coated glass. This produces a network-like metal phase embedded in a glass matrix. This metal phase is formed, for example, of thin platinum metal layers.
From German DE 196 36 493 C1 it is known to seed glass powder with a noble metal salt which is chemically bound to the glass powder surface. From this source it is also known to furnish such a seeded glass powder with a surface metallization having a typical thickness in the nanometer range. From DE 196 36 493 C1 a method is also known for seeding and coating of the metal powder with such a surface metallization.
Finally, temperature sensors are currently being made, in which platinum in the form of a thin layer is vapor-deposited onto a ceramic substrate, the vapor-deposited platinum layer then being trimmed by a laser to the desired resistance value, and being protected by a covering glass layer for use in an exhaust gas. The actual measuring of temperature, using such a temperature sensor, is based on the temperature dependence of the specific electrical resistance of platinum.
Compared to the related art, the sensor element according to the present invention has the advantage that a temperature sensor can be made from it which is mechanically robust, more cost-effective, and usable, for example, in exhaust gases at temperatures up to 1100xc2x0 C. At the same time, this temperature sensor demonstrates the same temperature sensitivity as in known platinum resistance temperature sensors, without requiring a costly thin film process for its manufacture, with subsequent overglazing.
Advantageous further refinements of the present invention result from the measures indicated in the dependent claims.
Thus, for realizing the desired temperature dependence of the manufactured temperature sensor, it is especially advantageous to carry out surface metallization in the form of platinum metallization having an average thickness of 0.5 nm to 10 nm, particularly 1 nm to 3 nm.
It is also advantageous that the glass ceramic fusion in the sensitive region of the sensor element according to the present invention can be applied by an ordinary thick layer process onto, for example, a customary ready-made aluminum oxide substrate, and fired in one step at a temperature of ca. 900xc2x0 C. What is particularly advantageous here is that, because of the adapted heat expansion coefficient of the glass ceramic fusion to the substrate, especially Al2O3, almost no thermomechanical stresses arise during use.
The design of the sensitive region in the form of a glass ceramic fusion, having a particularly network kind of metal phase embedded in it, further has the advantage that one may do without otherwise customary sintering temperatures of over 1400xc2x0 C. In the related art, such high temperatures are required when a ceramic, such as Al2O3 or zirconium dioxide is used, mixed, for instance, with platinum powder, instead of a glass ceramic fusion. Such ceramic powders for forming a desired resistance having platinum characteristics also have the disadvantage, as compared to the glass ceramic fusion used according to the present invention, that platinum layers produced at more than 1400xc2x0 C. often coagulate, and break up previously formed paths of conduction which guaranteed sufficient electrical conductivity, so that no material having a metallic resistance characteristic can be obtained.
The specific electrical resistance of the produced sensor element can finally be advantageously set by the thickness of the produced surface metallization of a component of the starting material for the glass ceramic fusion.
It is also advantageous that the starting material for producing the glass ceramic fusion product can be melted down at 850xc2x0 C. to 950xc2x0 C., refractory phases developing after crystallization, which are then stable at temperatures up to greater than 1000xc2x0 C. and adapted to Al2O3 in their heat expansion coefficient. The glass ceramic fusion product thus produced is, furthermore, electrically insulating even at high temperatures, provided a surface metallization of the component of the starting material is not used. Thus, it has, for example, an electrical breakdown resistance of more than 10 kV/mm at 800xc2x0 C.
For electrical contacting and for producing supply lines to the glass ceramic fusion product, a glass ceramic fusion product can also be advantageously used, however, having a modified composition, and thus having modified resistance characteristics. Alternatively, a conventional, low ohm electrode paste is also suitable for developing the supply lines and contacting.
In order to thermally decouple the temperature-sensitive area of the sensor element from the substrate lying below it, it is further advantageous to provide an intermediate layer between the glass ceramic fusion product and the substrate. For that purpose, preferably an intermediate layer of glass ceramic is used, having the same composition as the starting material for the glass ceramic fusion product in the sensitive area of the sensor element, however, doing without a surface metallization of a component of this starting material. In this respect the intermediate layer is electrically insulating.
Using such an intermediate layer has the advantage that thereby, at least to a great extent, measuring value falsification of the sensor element""s measuring values, caused by heat discharge from the sensitive area in the direction of the substrate, can be avoided. This happens because the intermediate layer has a substantially lower heat conductivity compared to the substrate made of Al2O3.